September 2016

Whole House Ventilation Systems

MVHR and central extract systems will be explained to you, during the course of this blog. By the end of this blog, you will have a good understanding to how both systems work.

Central Extract Ventilation

Central extraction ventilation, also known as MEV, is a whole house ventilation system.

They consist of a central extract unit. The unit is often stored in a loft space or a cupboard. The unit is then ducted to each wet room in the house. A typical wet room is a kitchen or bathroom. Each wet room will have a ceiling valve, which acts as the point of extraction.

MEV systems adjust the level of ventilation based on changes to the quality of air in the dwelling. For example, the unit will increase its ventilation if it detects increased levels of moisture in the air. This ensures that excess moisture is removed from the room and minimises the risk of damp and mould.

MEV is a proven low energy and efficient way of ventilating a property.


MVHR stands for Mechanical Ventilation with Heat Recovery. MVHR involves recovering the heat energy that is otherwise wasted from the extracted air. The heat is created from everyday life, such as cooking and bathing. In conventional ventilation systems, this heat is wasted, as it is removed from the home.

Heat recovery systems are different. This heat is recovered via an efficient heat exchanger. UK law dictates that any MVHR unit must have a heat recovery efficiency of 70% or higher. The recovered heat is then used to warm the incoming supply air. The supply air is then filtered and then supplied around the home.

MVHR units are now viewed as the best whole house ventilation by experts.

One of the reasons is due to the presence of an air filtration system. Depending on the quality of the filter used, an MVHR unit can filter out pollutants as small as pollen. This is ideal for those with allergies, as the air quality is healthier than it otherwise would be.

Furthermore, they can reduce the property's yearly heating bills. Depending on how efficient the heat exchanger is, among other factors, the unit can save you up to £350 a year. This is due to the reduced time needed to heat the property, as less heat is leaving the building.

Another feature that is commonplace for MVHR units is the summer by-pass facility. There are 2 types of summer by-pass facility:

  • Thermal - the supply fan slows when the indoor temperature gets too high. This minimises the amount of heat recovered. In turn, there is a reduction in the risk of overheating within the property. Although, there is still a chance that heat recovery could take place.
  • 100% - a damper causes the supply air to bypass the heat exchanger. This prevents unnecessary heat recovery taking place. This means the supply air remains at an ambient temperature. As such, you are able to maintain a comfortable living environment during the summer.

Using MVHR for Social Housing

Low income families are typically more affected by air pollution with children most at risk. The UK currently has over 40,000 deaths a year linked to poor air quality costs the economy billions a year. With indoor air pollution often 50% worse than outdoors and can contain up to 900 different pollutants it is imperative that a suitable system is installed to improve the health of the occupants as well as protecting the fabric of the home itself.

Mechanical Ventilation with Heat Recovery (MVHR) is the process of continually preheating filtered, incoming fresh air from outside by warming it with the outgoing waste air which is created by kitchens, bathrooms, utility rooms etc. Once the warmth is transferred, the now warm, clean air is the supplied to the living areas such as living rooms and bedrooms.

The following example will show the cost benefit of having a MVHR system installed in the property. This example assumes that both properties are 300m2 in size.

House AHouse B
Uses extract and trickle vents to ventilate the property. It has 2 air changes an hour and uses gas heating to warm the propertyUses an MVHR unit with 71%* heat recovery. It has an air change rate of 320m3/hr (89 l/sec), assuming no infiltration and uses gas heating to warm the property
Heat loss from infiltration at 2 ach 14,164 kWh per year at a cost of £0.0359 per kWh = £508.49 per yearHeat loss due to ventilation at 320m3/hr at 7677kWh with 71% thermal efficiency = 2226 kWh. With a cost of £0.0359 per kWh = £79.92 per year
Cost of running a bathroom and kitchen fan operating at 1 hour per day at 0.05kW = £2.08 per yearMVHR with two fans running, operating 24 hours a day at 693 kWh = £104.70 per year
Total ventilation energy cost = £510.57 per yearTotal ventilation energy cost = £184.62 per year

Additional saving with 80% thermal efficiency = £16.54

Additional saving with 90% thermal efficiency = £34.90

All new Airflow domestic MVHR units have a thermal efficiency of 90% or greater so you stand to see up to a £355 per year reduction in your energy bills.**

*Domestic Building Services Compliance Guide Section 8 specifies a minimum thermal efficiency of 70%

**assuming a kWh price of £0.0359

An ideal MVHR system to be used in social housing settings is the Adroit

Airflow Adroit MVHR units can be set up during the commissioning process to limit the tenants’ access to the control system. This removes the issue of tenants changing the ventilation rates for the property as well as them switching the unit off, guaranteeing effective ventilation always takes place within the property; protecting the health of both property and tenants. Your tenants, however, will still be able to tailor their ventilation around their daily routine through selecting the relevant ventilation profile to suit their needs.

As landlords, you would be able to remotely monitor the performance of the unit and be notified of when maintenance, such as changing the air filters, is required and in turn prolong the lifecycle of your Adroit unit.

Furthermore, in our Adroit DV50 and DV80 models, you can have these installed above the entrance to the property, so maintenance can be undertaken to the unit without disturbing the tenant.